In this particular scenario, and I believe only at the company that I'm working we have that. See below:
Win7 and Office 2010 + Lync Client 2013 (we have a Lync 2013!)
In that scenario after moving the Exchange mailbox (on-premises) to a new Exchange Online environment all users cannot see the meeting icon in their Lync Client.
That happens because the EWS cannot contact the properly both environment.
• As a workaround changed the default URL to use HTTPS instead:
• With this key set the calendar icon appears and EWS status shows OK in Lync configuration information
That key already exist in Office2013! :)